A process for determining characteristics of an electrochemical substance (electrochemically detectable substance) in a gas sample has become known from DE 43 44 196 C2.
The percentage of the electrochemical substance in the gas sample is calculated in the prior-art process from an integration over the total area enclosed between a reference line and a maximum of a measured physical variable. Partial areas are used for the calculation as partial integrals of the total integral of the total area, from which different characteristics of the substance to be detected, e.g., the concentration percentage in the gas sample or the species of the substance can be determined.
The electrochemical substance is recognized from the comparison of the partial areas, which are formed by the pattern of the curve of a measured electric variable of the measuring cell and the time. The presence of additional chemical substances leads to a slower course of the electrochemical processes. As a result, the profile of the measured electric value over time is changed. The ratio of the area percentages also changes as a result.
It was found in the above mentioned process that aging of the measuring cell as well as a reduction of the temperature lead to an increase in the internal resistance of the measuring cell. As a consequence of this, the electrochemical processes take place more slowly.
Thus, the pattern of the curve of the measured signal changes in the prior-art measuring cell with increasing aging of the measuring cell. Thus, the curve profile becomes flatter and broader in the course of the use time. A similar change in the pattern of the curve of the measured signal also occurs in the presence of additional electrochemical substances. A flatter curve profile may thus result from aging or a lower temperature of the electrochemical measuring cell, but also from the presence of additional electrochemical substances.